IND had been a long standing focus city dating back to the NW days. I suspect it’s now at the point where DL is happy with their business and capacity there, though.

I think it’s worth reiterating this is just an internal label. Once upon a time, the “focus city” designation mattered because it determined cities where connections were permitted in routing rules in fares. But now, DL almost always publishes specific constructed routes, and those lists of cities don't match the designations anymore. (For example, LAS sometimes shows up as a permitted connecting city, but AUS and RDU rarely seem to.)